Speaker
Description
Runaway electron (RE) currents of several mega amperes are expected to be generated in ITER disruptions due to avalanche multiplication. An uncontrolled loss of these high-energy electrons to the plasma facing components might cause serious damage. We present here observations of the RE-related relaxation phenomena during different phase of disruptions in the EAST tokamak.
RE formation phase: Electrostatic fluctuations are observed at the beginning of the current quench, which is detected by the HXR signals and related to strong RE losses. Note that the fluctuation is found in the disruptions triggered by amount of argon and neon injection but not helium injection, suggesting that the fluctuation should be driven by REs generated during disruptions. The frequency of the mode, in the range from 20 to 40 kHz, strongly depends on the amount and species of injected impurity and its evolution is suggested to be consistent with the evolution of REs. Mode structure of (m, n=1, 0) has been identified based on SXR arrays. GAM can capture most of the features of the fluctuations and barely-trapped/passing electrons can contribute to excite the mode.
RE plateau phase: Burst-like relaxations during the RE plateau phase cause large RE losses, which is seen by the spikes in the signals of magnetic coils. Two distinct types of RE-related relaxation phenomena are distinguished on the basis of the amplitude of magnetic fluctuations. Large-amplitude magnetohydrodynamic activity with indications of RE loss covering the entire energy range is observed during the RE plateau when the edge safety factor decreases to less than 3, and the external kink mode is discussed to resolve this anomaly. Burst-like relaxations with small-amplitude magnetic fluctuations and ~0.6 kHz frequency are confirmed from the spikes in the hard X-ray array signals under a negative loop voltage, and REs with medium energy are significantly lost at the same time. A possible mechanism for the instability is that due to the negative loop voltage, electric field de-accelerates REs and decrease the energy in the medium energy region, and finally, the modification of RE energy spectrum excites this kinetic instability.
These results will further deepen the understanding of RE losses in EAST and be an important part of RE mitigation or avoidance research in future.
| Speaker's title | Mr |
|---|---|
| Speaker's email address | tian.tang@ipp.ac.cn |
| Speaker's Affiliation | Institute of Plasma Physics, Chinese Academy of Science, Hefei |
| Member State or IGO | China, People’s Republic |
